The most common orthopedic condition for which professional medical treatment is sought is lower back pain. Although many factors may be responsible for causing lower back pain, a principal factor is damage or degeneration of an intervertebral spinal disc resulting in impingement on the nerve system, specifically the spinal cord, located within the spine. Such impingement may result in, for instance, loss of mobility, urinary and fecal incontinence, and sciatica or pain experienced in the extremities.
Damage to or degeneration of a spinal disc can result from a number of factors such as abuse or age. The disc itself is composed primarily of an annulus and a nucleus contained therein. The annulus is a fibrous annular piece that attaches to the adjacent vertebrae and contains the nucleus, which is in turn a gel-like viscous material capable of shock absorption and flowable to permit poly-axial rotation and resilient compression of the vertebrae and spine. Most frequently, disc degeneration results from damage occurring to the annulus such that the flowable nucleus material may leak or seep out of the annulus. Disc degeneration also can occur in other ways, such as by being deprived of nutrient flow leading to a dried disc susceptible to damage. Because the nuclear material is flowable, extensive damage to the annulus is not necessary for leakage to occur.
Currently, approaches to treatment of spinal problems directly affecting the spinal cord are numerous. For instance, immobilization and high doses of corticosteroids may be employed. The dominant surgical procedures for treatment of these problems are spinal fusion and discectomy. Fusion is a method where adjacent vertebrae are immobilized so that they permanently secure to each other by having bone growth between and to the vertebrae, while discectomy involves removal of a portion or an entirety of a spinal disc.
However, the current practice of each of these procedures typically has certain limitations. With fusion, making a portion of the spine generally rigid produces a reduction in mobility, and drastically alters normal load distribution along the spinal column. Due to these factors, the non-fused portions of the spine experience stress and strain that are significantly increased over normal physiological motions. The increased stress and strain on the non-fused portions may lead to accelerated disc degeneration of the non-fused portions, particularly the adjacent levels of the spine.
Discectomy is effective for relieving sciatic pain by removing the damaged or herniated disc tissue compressing the spinal nerves. However, current discectomy often may lead to a reduction of the disc space between adjacent vertebrae, as well as instability in the affected portion of the spine. Such long-term effects with current discectomy often result in further surgery several years after the initial discectomy surgery.
A recent, though not new, development for spinal surgery of this type is a procedure known as disc arthroplasty for restoring or reconstructing the disc using a prosthesis to replace a portion or entirety of the damaged disc. The primary objective of disc arthroplasty is to restore or maintain the normal disc anatomy and functions, while addressing and treating the causes of the pain. However, little success has been experienced with prosthetic disc implants due to the complexity of the natural disc structure and biomechanical properties of a natural spinal disc. As used herein, the term natural refers to normal tissue including portions of the spine and the disc.
Two types of prostheses for disc arthroplasty are currently believed to merit further development by medical science and research. One type is a total disc prosthesis, or TDP, where the entire spinal disc is replaced after radical discectomy. A typical TDP includes structures that together attempt to mimic the properties of a natural disc.
The other type is a disc nucleus prosthesis, or DNP, that is used to replace only the nucleus of a spinal disc after a nucleotomy while retaining the annulus of the disc and, possibly, the end plates intact. As discussed above, failure of the natural disc does not require extensive damage to the annulus, and the annulus would often be capable of retaining a non-flowing prosthetic nucleus. Implantation of a DNP involves clearing of the natural nucleus from the annulus through the procedure known as nucleotomy, and inserting the DNP within the annulus. Accordingly, DNPs are typically smaller and require less extensive surgery than TDPs while still mimicking some of the biomechanical properties of a natural intervertebral disc. Herein, the term artificial disc device or implant can refer to either a TDP or a DNP.
In using disc implants, one problem relates to the preparation for the surgical procedure for implanting either the TDPs or DNPs. The time required for preparing for surgery, and specifically preparing the implants and inserters for use, can be important for both patient welfare and in terms of cost efficiency. For instance, if only one of the ends of the implant is configured for gripping by an inserter tool, this requires the medical personnel to locate the proper end of the implant and then connect it to the inserter. Extra time is wasted when an implant has otherwise similarly configured ends such that it is difficult to easily determine which end of the implant attaches to the inserter. The problem is compounded when the implant has multiple components (such as a top and bottom portion), and the medical personnel need to first properly match the disc components to each other so that the ends of each component configured to connect to each other are properly aligned with each other before attachment of the disc to the inserter. This can waste time during preparation for the surgical procedure. Accordingly, an artificial disc would be desirable that has portions that do not only connect with each other in one configuration and require that the disc be mounted on an inserter tool in a single orientation.
Other improvements specifically for the DNP procedure would be desirable. As mentioned above, a DNP requires less extensive surgery than for a TDP since it replaces only part of the disc. Implantation of most known DNPs with pre-formed dimensions generally requires a 5-6 mm, or larger, incision in the annulus for implantation. The incision, however, should be kept as small as possible to hold the DNP within the annulus without using anchors on the DNP that extend into the end plates of the vertebrae for securing the DNP. The minimal invasiveness of the procedure results in minimal recovery and post-surgical pain, and interbody fusion remains a viable revision surgery. Thus, maintaining a small incision and keeping damage to the annulus to a minimum is a high priority. Therefore, it would be desirable to provide a DNP and inserter that does not require an enlarged incision and does not significantly damage the annulus or other tissue during insertion and placement of the DNP.
Another problem with DNP structure and the surgical procedures involving DNP relate to the positioning of the artificial disc within the nuclear space. For some DNPs, once the implant is positioned in the nuclear space, it must be rotated in order to position it properly for providing its full range of motion and its full shock absorption capabilities to the patient. Thus, a DNP and an inserter that manipulates the DNP within the nuclear space without causing damage to the annulus are also desired.
As mentioned above, it is desirable that the implant match the size and shape of the natural disc because the implant may cause damage to the surrounding anatomical structure such as the annulus and may cause pain to the patient if the implant is too large or too small. Since the process of inserting, positioning and then releasing the implant can be complex, especially when an implant has multiple pieces, known sizing tool are used for measuring the nuclear space so that an implant that fits the shape and size of the nuclear space can be chosen before it is inserted into the nuclear space.
In order to determine the size of the nuclear space, conventional sizing tools are used such as a set of trial spacers disclosed by U.S. Pat. No. 6,478,801. In such a set, each spacer has a different size and is sequentially inserted into the nuclear space and then removed in trial and error fashion until the trial spacer fits the nuclear space which indicates the correct size of implant that should be used. The spacers may also be used to distract and stretch the tissue surrounding the nuclear space to make it easier to insert the implant. With some known spacers, however, connecting and disconnecting the spacer from the insertion tool that holds the spacer can be a cumbersome and time wasting process during the surgery while the patient is under anesthesia. This occurs when a surgeon is limited to tightening and loosening one or more fasteners to secure and unsecure the spacer to the insertion tool on each trial.
Other problems such as those mentioned above regarding proper orientation of the implant within the nuclear space also arise when attempting to measure the size of the nuclear space with trial spacers. In order to obtain an accurate measurement, some known spacers, similar to the implants, have elongated shapes to imitate the size and geometry of a natural disc, and in turn the artificial disc to be implanted, and the nuclear space to obtain a more accurate measurement. Thus, in order to minimize the size of the incision on the annulus surrounding the nuclear space to be measured, the spacer is mounted on an insertion tool with its longitudinal axis parallel to the longitudinal axis of the insertion tool on which it is mounted. The spacer is then inserted through the incision with its narrow end as the leading end of the spacer and facing the incision. With this configuration where the longitudinal axis of an elongated spacer is parallel to the longitudinal axis of the insertion tool, however, the surgeon is limited to more lateral surgical approaches (relative to the posterior-anterior direction) in order to orient the spacer with its longitudinal axis extending transverse to the anterior-posterior direction to imitate the orientation of the natural disc within the nuclear space. Thus, it would be desirable to have an insertion tool for inserting a spacer in a nuclear space that is not limited to certain surgical approaches.